Well, it looks like Terra is performing some kind of powerful attack. Terra doesn't have a magic meter. I'd assume that more basic attacks and magic don't use up any of the gauge, but more powerful attacks and magic do.
But, then again, the battle system of BBS has been rather hard to decipher from the screens we've seen thusfar.
Yeah, I'd say it's one of those big magic amplified BOOM! moves. Wonder what it does when unleashed. :toungesmile: